The intermediate bus power architecture is widely used in electronics devices to provide multiple output voltages (for example, 3.3V, 1.8V, 1.2V) from a centralized input voltage (for example, 48V), to achieve a flexibility of designing the power system, and to overall enhance a power conversion efficiency. As shown in FIG. 1, an isolated DC to DC converter, called the intermediate bus converter, is employed to convert the input voltage to a low voltage, and then a non-isolated DC to DC converter provides multiple output voltages for satisfying various requirements of the system based on the converted low voltage.
Most of the intermediate bus converters are installed on a system board of the user. There are a variety of user systems such as network systems, computer systems, server systems, and communication systems. Such converters are integrated with a master power switch, a synchronous rectifier, a transformer, an inductor, etc., wherein the master power switch and the synchronous rectifier are installed on one main PCB board. The magnetic cores respectively from the front surface and the back surface of the PCB board are clasped so as to constitute the transformer together with the PCB winding, and the magnetic cores of an inductor are installed to constitute the inductor in a similar manner. Since all the power flow will pass through the intermediate bus converter, the efficiency and power density of the intermediate bus converter is particularly important for improving the overall power system efficiency and optimizing the system thermal design. For the isolated DC to DC converter, the power loss may come from several reasons: primary switch loss including switching loss and conduction loss, transformer power loss including magnetic core loss, winding loss and leakage inductance loss, and secondary rectifier power loss including conduction loss and reverse recovery loss. Moreover, the power loss of the body diode of the synchronous rectifier usually significantly impacts the efficiency of the converter due to its poor reverse recovery characteristic.
In order to improve the efficiency of the converter, the designer may keep the balance of the above power loss portions, and keep the total amount of power loss as low as possible. There are several solutions to reduce the power loss of the intermediate bus converter. The first solution is employing a resonant converter, as shown in FIG. 2, the circuit topology of which includes primary switches S11 and S12; primary capacitors C1, C2 and C3; a primary inductor L1; a transformer T; secondary synchronous rectifiers S21 and S22; and a secondary capacitor C4. The resonant topology keeps the primary switches S11 and S12, and the secondary synchronous rectifiers S21 and S22 operating in a soft switching condition, so the primary switching loss and secondary reverse recovery loss will be eliminated. And the resonant topology may use a lower voltage rating synchronous rectifier in the secondary side of the transformer because it is not necessary to configure the output inductor, which ensures the continuous output current in the PWM converter. However, the resonant topology will cause a high RMS current both in the primary side and the secondary side, and result in high conduction loss. Another issue of the resonant converter is that the large output current ripple may cause voltage fluctuation if the capacitance value of the output capacitor is not enough. Therefore, it is not a good choice if we need take more consideration on the output current ripple.
For the PWM intermediate bus converter, one method to improve the efficiency is removing the regulation function, as shown in FIG. 3. The converter without regulation, usually called non-regulated converter, operates in a fixed duty cycle, which is close to 50%. In addition, both peak current and RMS current in the primary switches are reduced, and both conduction loss and switching loss of the primary switches are reduced accordingly. Also, due to the above fixed duty cycle, the voltage stress of the synchronous rectifiers is reduced, so the lower voltage rating synchronous rectifier may be used. The conduction loss and reverse recovery loss of the synchronous rectifiers are both reduced since the lower voltage switches usually have a lower on-resistance and better reverse recovery characteristic. However, in the related art, in order to keep the minimized switch loss in the secondary current loop, two secondary transformer windings are usually adopted, and the two secondary windings conduct the current alternatively in the positive and negative switching cycles, as shown in FIG. 3. Although the conduction loss of the synchronous rectifiers is reduced due to only one switch in the secondary current loop, the power loss in the transformer winding is still high since the two secondary windings conduct current sequentially and alternatively. In this way, the utilization rate of the transformer secondary windings is relatively low, and the conducting impedance and the conducting loss of the transformer secondary windings are very high. In addition, the two secondary windings need three high current terminals, and the power loss from current terminals is relatively high.
In general, the non-regulated converter is only used in a narrow input voltage system, since the output voltage varies with the proportion of the input voltage. Once the input voltage range becomes wider, or the non-isolated DC to DC stage can't tolerate a wider input range, the regulation in the intermediate bus converter is still expected. Therefore, an improved circuit design and configuration to achieve higher power conversion efficiency is desired for both the regulated converter and the non-regulated converter.
The information described above is only used to enhance the understanding of the background of the present disclosure, and thus may include the information which is not regarded as the ordinary skill in the art for the person skilled in the art.